A known apparatus is described, for instance, in U.S. Pat. No. 3,709,432. This document discloses a so-called continuous jet printer for printing materials using a first droplet ejection system arranged to generate a continuous stream of first droplets from a fluid jetted out of an outlet channel. During the exit of the fluid through an outlet channel, a pressure regulating mechanism provides, with a predetermined regularity, variations in the pressure of the viscous fluid adjacent the outflow opening. This leads to the occurrence of a disturbance in the fluid jet flowing out of the outflow opening. This disturbance leads to a constriction of the jet which in turn leads to a breaking up of the jet into drops. This yields a continuous flow of egressive drops with a uniform distribution of properties such as dimensions of the drops.
The publication shows a gas jet mechanism to selectively deflect the drops. The fluid jet length is controlled of droplets generated by the regulating mechanism. The deflection properties of the droplets differ from that of the jet, so that droplets can be selectively deflected.